Analyze Motion intermittently does not work

I have the latest up-to-date version of Motion 4 on a 2.66ghz Dual Intel Xeon Mac Pro running Mac OS 10.6.5 with 8gb of ram.

I have been working on a project now for a few months that I have been using Motion and the Analyze Motion behavior extensively. There are times when I apply the AM behavior and click on the analyze button and it does not do anything. Everything else seems normal except it won't analyze. I tried the match move behavior to analyze with no success either. I restarted the computer, deleted all the other effects in the project file so the only effect was the analyze motion behavior, trashed prefs, etc. I guess the next step would be to reinstall Motion, but I would like that to be the last step. There must be something that sets it off, because I will move on to something else, and when I get back to it a couple hours, sometimes days, later, it will work just fine.